This is a modification of the Layan Dark theme I made based on the GTK & KDE theme Layan by pling.com user vinceliuce. I just changed the highlight color to #198cff, set background tabs to be transparent, and changed the background image to this moon over the sea one.
There's a story behind the background image that's kind of strange. I once sketched a moon over the sea inspired by what I imagined when listening to an instrumental track called "Aurora Borealis" by the artist Port Blue. Fast-forwarding in time, I had downloaded the Pale Moon web browser and saw on its start.me homepage this image which looked nearly exactly like the scene I imagined. I tracked the image down and popped it into Vivaldi.
I first found the image here using Bing image search: https://1freewallpapers.com/nightfall-mountain-sea-moon/de
The earliest page I can find with the image is this one: https://www.bwallpapers.com/wallpaper/nightfall-mountain-sea-moon-9839/
I have no clue who the image's author is, but the first website listed (which I downloaded it from) says the image is only for personal use, with commercial use not allowed. I checked the image rights box on Vivaldi's theme uploading page with skepticism, since I'm not sure where the original image comes from and am therefore uncertain if I have permission to use it this way.